text: Assuming that a vessel is cylindrical with fixed cross-sectional area , with orifice of area  at the bottom, then rate of change of water level height  is not constant. The water volume in the vessel is changing due to the discharge  out of the vessel:
  Integrating both sides and re-arranging, we obtain
  where  is the initial height of the water level and  is the total time taken to drain all the water and hence empty the vessel.
  This formula has several implications. If a tank with volume  with cross section  and height , so that , is fully filled, then the time to drain all the water is
  This implies that high tanks with same filling volume drain faster than wider ones.
  Lastly, we can re-arrange the above equation to determine the height of the water level  as a function of time  as
  where  is the height of the container while  is the discharge time as given above.